1. Field of the Invention
The present invention relates generally to an optical waveguide fiber designed for single mode operation over an extended wavelength range, and particularly to such a fiber having a core size and bend resistance comparable to step index single mode optical waveguide fiber.
2. Technical Background
Delivery of broadband telecommunication capability to a local area, such as an office building or residential neighborhood, presents a different set of requirements in comparison to those for longer distance telecommunications, for example interstate or intra-city loop telecommunications. Local area broadband telecommunications, also called local access or home access broadband telecommunications, is characterized by a multiplicity of home or local nodes located relatively close together, each of which must be connected into a main telecommunication line leading to a distribution node, which is typically called the central office. The distribution node serves as a hub that carries information back and forth between each of the local nodes and the service providers, offering such services as interactive television, voice communication, or facsimile communication. The high concentration of nodes increases the impact of splicing and connecting on system installation, span length as limited by span attenuation, and maintenance cost. In addition, the cost and reliability of signal sources gains in importance. This is especially true of the sources configured to transmit signals from the plurality of local nodes back to the distribution node.
The concept of selecting the parts of a telecommunication system in order to optimize its operation has successfully been applied to longer distance telecommunications systems. Optimization refers to reaching a balance among such factors as system cost, system reliability, and quality of the services provided. An example of optimization of longer distance systems is compensation of optical waveguide fiber total dispersion by using optical waveguide fibers having different sign of total dispersion relative to one another. This strategy has successfully extended the distance between electro-optical regenerators in such systems. Reducing the number of regenerators has decreased system cost and improved system reliability.
In intra-city or metro applications, which typically require an increased number of nodes in comparison to an inter-city link, ways have been sought to decrease the cost of each node while maintaining quality of service, i.e., signal integrity. Directly modulated distributed feedback lasers used in combination with negative dispersion optical waveguide fiber results in the waveguide fiber compensating the positive chirp of the laser. The system is optimized because the proper combination of signal source and fiber results in signal pulse compression.
In typical compensation strategies, the system components are engineered to compensate one another so that signal quality is maintained while reducing the required number of regenerators.
In the present instance, the case of local access, vertical cavity surface emitting lasers (VCSEL) have been identified as desirable sources for use in the home access nodes. The VCSEL is inexpensive and reliable. An additional desirable feature of the VCSEL is its large emitting area which provides for easier alignment, i.e., optical coupling, of the laser and an optical waveguide fiber. However, the cost of the VCSEL increases dramatically for laser wavelengths above 1000 nm. Thus the optical waveguide fiber low attenuation window around 1300 nm or 1550 nm is not compatible with a low cost VCSEL. The low attenuation window around 850 nm is compatible with a low cost VCSEL, thus motivating a search for a suitable optical waveguide fiber designed to operate around the 850 nm wavelength window.
Multimode optical waveguide fiber having, at a wavelength of 850 nm, bandwidth above 500 MHz-km and attenuation less than 0.4 dB/km has been available for several years. However, the use of multimode fiber in the home access node can be expected to result in signal degradation due to modal noise, especially in the case where a narrow-band laser source, such as a VCSEL, is used.
The increase in number and complexity of services to the home will place an emphasis on achieving broadened operating wavelength bands. In particular, demand will increase for expansion into the attractive operating window at wavelengths in the range above 1300 nm, particularly in the operating wavelength windows around 1310 nm and 1550 nm. At these higher wavelengths, a Fabry-Perot cavity laser can advantageously be used as a signal source provided the total dispersion of the optical waveguide fiber is suitably low. The Fabry-Perot laser is typically reliable and low in cost, but has a relatively wide spectral width. The use of this laser type is appropriate in system portions of shorter span length operating at the wavelengths of higher fiber dispersion or at wavelengths near the zero dispersion wavelength of the optical waveguide fiber. An optical waveguide fiber having properties that can accommodate both a VCSEL laser source as well as a Fabry-Perot laser source would offer a desirable degree of flexibility in local access system design. For example, VCSEL laser sources operating in a wavelength band near 850 nm could be used within a home or office building. The Fabry-Perot laser sources operating near a zero dispersion wavelength in the vicinity of 1310 nm could be used for transmissions originating outside the residence or office where longer span lengths are typically used.
Single mode optical waveguide fiber having a cut off wavelength less than 850 nm has been manufactured. To achieve this lower cut off wavelength, the numerical aperture or the core diameter must be reduced. This reduction results in increased difficulty in making optical connections, for example, splice or coupling connections, and in weaker guiding of signals at wavelengths far from cut off, such as signals in the operating windows around 1310 nm and 1550 nm. Many of the broadband telecommunications systems in use today operate at either or both of these higher wavelength windows.
An example of a step index single mode optical waveguide fiber in accord with the prior art is illustrated in FIG. 1. Step index 2 illustrates a step index single mode optical waveguide fiber having a relative refractive index percent, Δ%, and radius 6 selected to provide a cut off wavelength of about 1200 nm. Typically, the core diameter is about 9 μm and the Δ% is about 0.3% to 0.4%. In order to alter the step index 2 of FIG. 1 to provide a cutoff of about 850 μm, the core diameter 6 must be reduced to core diameter 9. In the illustration, the reduction in diameter to achieve the lower cut off wavelength is about 20%. The lower value of core diameter increases the difficulty of making optical connections with components or other fibers in a telecommunications system.
Thus there is a need for a single mode optical waveguide fiber that has a cut off wavelength less than about 850 nm but yet guides light well at wavelengths around the 1310 nm and 1550 nm operating windows. An additional desired property of such a single mode optical waveguide fiber is that it has core diameter sufficiently large to facilitate the making of optical alignments and connections. Also, an optical waveguide fiber property which grows in importance in the part of a communication system that reaches into individual homes or office buildings is resistance to bend induced loss over an extended wavelength band.